Function perform_relmap_update

src/backend/utils/cache/relmapper.c:964–1007

* Merge the specified updates into the appropriate "real" map, * and write out the changes. This function must be used for committing * updates during normal multiuser operation. */

 * updates during normal multiuser operation.
 */
static void
perform_relmap_update(bool shared, const RelMapFile *updates)
{
	RelMapFile	newmap;

	/*
	 * Anyone updating a relation's mapping info should take exclusive lock on
	 * that rel and hold it until commit.  This ensures that there will not be
	 * concurrent updates on the same mapping value; but there could easily be
	 * concurrent updates on different values in the same file. We cover that
	 * by acquiring the RelationMappingLock, re-reading the target file to
	 * ensure it's up to date, applying the updates, and writing the data
	 * before releasing RelationMappingLock.
	 *
	 * There is only one RelationMappingLock.  In principle we could try to
	 * have one per mapping file, but it seems unlikely to be worth the
	 * trouble.
	 */
	LWLockAcquire(RelationMappingLock, LW_EXCLUSIVE);

	/* Be certain we see any other updates just made */
	load_relmap_file(shared, true);

	/* Prepare updated data in a local variable */
	if (shared)
		memcpy(&newmap, &shared_map, sizeof(RelMapFile));
	else
		memcpy(&newmap, &local_map, sizeof(RelMapFile));

	/*
	 * Apply the updates to newmap.  No new mappings should appear, unless
	 * somebody is adding indexes to system catalogs.
	 */
	merge_map_updates(&newmap, updates, false);

	/* Write out the updated map and do other necessary tasks */
	write_relmap_file(shared, &newmap, true, true, true,
					  (shared ? InvalidOid : MyDatabaseId),
					  (shared ? GLOBALTABLESPACE_OID : MyDatabaseTableSpace),
					  DatabasePath);

	/* Now we can release the lock */
	LWLockRelease(RelationMappingLock);
}

/*
 * RELMAP resource manager's routines